This invention relates to a novel method for testing the electrical breakdown characteristic of the hermetic seal between the panel and the funnel of a completed cathode-ray tube.
In one popular design of a color television picture tube, which is a type of cathode-ray tube, a glass faceplate panel is sealed to a conical glass funnel with a sealing layer of devitrified glass. The sealing layer is produced by applying a layer of a ground glass or frit to the seal land of the funnel, then the panel is placed on top of the funnel with the panel seal land against the frit layer, and then the assembly is heated until the frit layer melts and wets the seal lands and forms the sealing layer. Subsequently, the assembly is cooled to room temperature, and the assembly of the tube is completed.
Since the tube carries very high voltages, it is necessary for the sealing layer to have a high breakdown voltage in addition to having hermetic qualities. An electrical breakdown in the sealing layer renders the tube inoperative. Such breakdowns can be the result of the failure to remove conductive material from one of the seal lands, or a partial failure of the molten glass to wet all of the two seal lands, or impurities in the frit, or other causes.
It has been the practice to test the electrical breakdown characteristic of the sealing layer between the panel and the funnel by applying a flexible or resilient band of electrically-conductive material against the external surface of the sealing layer, electrically grounding the band through a grounding circuit, applying a substantially higher-than-operating testing voltage to the internal funnel coating through the anode button of the tube, and sensing a surge in leakage current in the grounding circuit if the sealing layer is defective. Contacting the band with the seal is degrading to both the sealing layer and the band. Also, relatively complex testing machines must be employed, for example, the apparatus disclosed in U.S. Pat. No. 3,943,438 to H. H. Barber et al.